The invention relates generally to digital audio broadcasting systems and, more particularly, to frame synchronization techniques in digital audio broadcasting systems.
The amplitude modulation In-Band-On-Channel (AM IBOC) Digital Audio Broadcasting (DAB) system, similar to the hybrid frequency modulation (FM) IBOC system, uses an orthogonal frequency division multiplexing (OFDM) scheme. In the OFDM scheme, a number of sub-carriers are modulated by a digital signal and then multiplexed together such that, in the time domain, adjacent symbols do not interfere with each other. To further minimize the inter-symbol interference, a guard period is added at the transmitter between each adjacent symbol-pair. Using a cyclic prefix or suffix in this guard period, one can determine OFDM frame boundaries. A combination of 32-QAM (quadrature amplitude modulation) and BPSK (binary phase shift keying) has been proposed as an approach for this system.
By way of an example of the use of a cyclic prefix or suffix, suppose that the useful symbol period contains 512 samples denoted as x0, x1, . . . , x511. A signal {yn} may be constructed in the following manner:       y    n    =      {                                                                      x                                  n                  +                  498                                            ,                              0                ≤                n                 less than                 14                                      ⁢                          xe2x80x83                                                                                                      x                                  n                  -                  14                                            ,                              14                ≤                n                 less than                 526                                      ⁢                          xe2x80x83                                                                                      x                              n                -                526                                      ,                          526              ≤              n               less than               540                                          
In other words, the first 14 samples of {yn} are equal in amplitude and phase to the last 14 samples of the useful symbol period. They are said to constitute the cyclic prefix. Similarly, the last 14 samples of {yn} are the same as the first 14 samples of the useful symbol period. They are said to constitute the cyclic suffix.
However, in the AM IBOC DAB system, since each symbol is passed through a pulse shaper for additional signal conditioning, the standard OFDM frame synchronization procedure based on the correlation property of the cyclic prefix does not work. The cause of this problem is as follows. Since the output of a filter is given by the convolution of the input signal with the impulse response of the filter, when the input is passed through a pulse shaping filter, the samples in the guard period are no longer equal to the corresponding samples in the useful symbol period.
There is another, more difficult problem with the AM IBOC system. To support a higher data rate of the digital signal, digital data is transmitted over frequencies not only outside of the analog host bandwidth, but also in the bandwidth occupied by the host. The host bandwidth is 10 kHz (kilohertz). The digital signal bandwidth is 20 kHz. The signal in the outer digital lobes is about 5 dB (decibels) higher than the signal under the host. To minimize the interference from the digital signal to the analog AM, the level of the digital signal in the xc2x15 kHz frequency range must be about 20 dB below the analog signal. Also, to be able to recover the digital signal in the xc2x15 kHz in the presence of the stronger analog signal, it is necessary for the digital and analog signals to be in quadrature with respect to each other. Since the latter requirement should be met for any arbitrary data pattern, BPSK should be used for the OFDM sub-carriers under the analog host.
Therefore, there is a need for methods and apparatus for performing frame synchronization in a DAB system, such as the AM IBOC DAB system, which do not rely on the use of the cyclic prefix or suffix, and which eliminate or at least reduce the effects associated with the shortcomings of the prior art as discussed above and which otherwise exist in the art.
The present invention provides a robust OFDM frame synchronization procedure that does not depend on the use of the cyclic prefix or suffix. The procedure is based on sending, in each symbol period, a known data pattern, for example, 1, over two OFDM sub-carriers, for example, k and xe2x88x92k. In one exemplary embodiment, 1xe2x89xa6|k|xe2x89xa636. It is to be appreciated that k may be selected arbitrarily, however, k is selected such that the sub-carrier frequency is within the host spectrum and computations become simpler. Also, more than two sub-carriers can be used. That is, the known pattern may be inserted into more than two sub-carriers and transmitted.
In any case, in a preferred embodiment, the two sub-carriers are modulated using BPSK. To obtain frame synchronization, we search the demodulated signal for the known data pattern by taking the Fast Fourier Transform (FFT) of the received signal at sub-carriers k and xe2x88x92k. Synchronization is achieved when the same pattern is detected in both sub-carriers. Once synchronization is achieved, a phase-locked loop is used to track the OFDM frames. If synchronization is lost as indicated by the phase-locked loop, we return to the search mode.
Advantageously, the procedure works in the presence of a wide range of additive white Gaussian noise (AWGN) and interference from the analog host. For example, synchronization may be achieved when the SNR (signal-to-noise ratio) is 23 dB or more and host interference is 20 dB above the digital signal.
Although the procedure is particularly well-suited for sub-carriers in the above range, it can be used with any sub-carrier over the entire xc2x110 kHz range and other data patterns. Similarly, the frame synchronization algorithm of the invention works for higher analog AM signal levels as well.